Our DFT training set consisted of molecular dynamics simulations of unit cell configurations (12 atoms total), run for 5 ps at 400 K with simulation cells initially optimized to pressures of \(-8\) , 0, 10, and 100 GPa. All MD calculations were run in the constant temperature and volume (\(NVT\) ) ensemble with Nosé-Hoover thermostat chains[1]}, [2]}, [3]} and a timestep of 0.2 fs. The slightly elevated temperature and wide pressure range including negative pressure were chosen in order to yield a broad sampling of the underlying potential energy surface. Atomic forces and the diagonal of the stress tensor were then sampled from MD configurations at fixed time intervals of \(\sim 160\)  fs in order ensure configurations were as statistically uncorrelated as possible. This yielded up to 30 MD snapshots for each pressure. In addition, in order to sample hyper- and hypo-coordinated configurations in the system, we included MD data for a unit cell with a single hydrogen interstitial or single vacancy site, each run for 5 ps. The starting configuration for these defect simulations were taken from the ground-state stoichiometric unit cell, with a single H atom removed or added in an octahedral interstitial site. This yielded a total of 153 unit cell-sized configurations for our training set.
